This present invention relates to optical performance monitors for use in optical communication systems.
In wavelength division multiplexed (WDM) optical communication systems, many different wavelength channels are simultaneously carried by a single optical waveguide. In dense wavelength division multiplexed (DWDM) systems, up to 80 channels with a 50 GHz (0.4 nm) channel spacing can be accommodated in the 1525 nm to 1575 nm wavelength range (C band).
Performance of these systems is verified with an optical performance monitor (OPM) by monitoring the wavelength, power, and signal-to-noise ratio (SNR) of each of the DWDM channel signals. For example, an OPM located at the receiver line terminal is important for network commissioning and long term monitoring. An OPM located at the transmitter end can perform the added task of wavelength locking. Between the terminals, an OPM can measure deviations from the predefined channel wavelengths due to drifts or instabilities. For example, the OPM can be used to measure the successful reconfiguration of a tunable add/drop multiplexer or the imbalances in a recombined DWDM signal resulting from the individual channels traveling disparate paths throughout the network.
Traditionally, optical performance monitoring has been accomplished with optical spectrum analyzers, which for example, achieve high optical resolution by using a monochromator. In general, these optical spectrum analyzers are limited by the slow scanning speeds necessary for achieving high resolution and sensitivity.
An example of an optical spectrum analyzer with a faster response time is based on a tunable Fabry-Perot etalon. A Fabry-Perot etalon has two partially reflective mirrors, or surfaces, facing each other and separated by a predetermined gap that forms a cavity. The etalon has a periodic response to a multi-wavelength input signal; namely, it only transmits certain wavelengths for which the cavity is said to be in resonance. The spacing between the certain wavelengths, or fringes as they are commonly called, is referred to as the free spectral range (FSR) of the cavity and is a function of the reflectivity and the spacing between the mirrors. Typically, the etalon is tuned by varying the spacing between the mirrors.
However, optical spectrum analyzers based on tunable Fabry-Perot etalons are limited by the periodic response of the etalon, which makes it difficult to uniquely determine the wavelength of interest, particularly in the presence of multiple wavelengths. A second disadvantage relates to the fact that a Fabry-Perot filter with high resolution is limited in free spectral range, i.e. it cannot be tuned over a wide range of wavelengths at a high resolution. In fact, the mechanical and optical requirements imposed on a tunable Fabry-Perot filter for achieving the required rejection, stability, and wavelength setting accuracy make such devices excessively costly.
It is an object of the instant invention to provide an optical performance monitor that obviates the above disadvantages.
The present invention relates to an optical performance monitor having a tunable grating sandwiched between the first and second reflective surfaces of an etalon. Individual channels within the optical data signal are monitored by tuning the grating over the wavelength range of the optical data signal to measure an intensity of each channel. As the tunable grating scans the wavelength range of interest, it simultaneously changes the resonant cavity length of the etalon, thus allowing the etalon to effectively monitor the tunable grating with a wavelength outside the wavelength range of the optical data signal wavelength. Advantageously, this optical performance monitor provides high sensitivity, while simultaneously monitoring an optical data signal.
Accordingly, the present invention relates to an optical performance monitor comprising:
a first port for launching an optical data signal including a plurality of wavelength channels within a wavelength range;
a second port for launching a reference signal having a wavelength xcex;
an etalon for receiving the optical data signal and the reference signal, the etalon having a free spectral range (FSR) and including a first partially reflective surface and a second partially reflective surface spaced apart by a length l for forming a resonant cavity for a signal having a wavelength substantially equal to xcex;
a tunable grating optically disposed between the first and second reflective surfaces of the etalon for successively reflecting one channel at a time from the optical data signal;
a first detector for measuring an output of the tunable grating; and
a second detector for measuring an output of the etalon.
Advantageously, the OPM of the instant invention has significantly lower cost considerations than the high quality, tunable etalons found in prior art OPMs.
Furthermore, relatively little splitting of the optical signal is required, thus requiring lower tap splits in the network. Since there is relatively low loss, a device in accordance with the present invention is optionally multi-functional. For example, the light extracted with a OPM in accordance with the instant invention may also be analyzed to determine polarization dispersion loss, etc.